The present invention relates to an initial core of a boiling water reactor (BWR).
It is necessary to increase the average enrichment of an initial core to raise the discharge exposure of the initial core. Therefore, in an initial core, which is loaded with a plurality of fuel assemblies having a different average enrichment, the difference in the nuclear characteristics between a high enrichment fuel assembly having a high average enrichment and a low enrichment fuel assembly having a low average enrichment becomes large.
In case a high enrichment fuel assembly and a low enrichment fuel assembly adjoin each other, a thermal neutron is caused to flow from the low enrichment fuel assembly with a high thermal neutron flux to the high enrichment fuel with a low thermal neutron flux. Therefore, because the output of the fuel rods of the high enrichment fuel assembly increases, and the MLHGR (Maximum Linear Power Heat Generation Ratio) and the MCPR (Minimum Critical Power Ratio) in the beginning of burnup become severe, an improvement in the thermal margin becomes a problem.
To improve the thermal margin, fuel rods containing gadolinia (hereinafter called a "Gd fuel rod") are arranged symmetrically in the cross section of a conventional fuel assembly. In case the average enrichment of an initial core is heightened for high burnup, control rods are inserted in the core for a long time, and then they are extracted to suppress excess reactivity.
Therefore, the output of one side near to the control rod is smaller than that of the other side far from the control rod in the cross section of a fuel assembly loaded into a control cell. Burnup of fuel rods on the one side near to the control rod is delayed. This is called control rod history effect. Due to the influence of this control rod history effect, the thermal margin could not be sufficiently secured in a conventional initial core. Particularly, in a case where the high enrichment fuel was loaded into the control cell in a second operation cycle, the thermal margin was severe.